Techniques have been worked out allowing isolation, characterization and quantitation of precursor cells to adipocytes in adipose tissue. By using these methods in combination with previously available techniques adipose tissue growth and development can be followed. Fat-feeding in the rat is known to cause an increase in adipose tissue mass. This is occurring by a series of events. First, fat for storage is packed into the available fat cells. There is now also an increase in the formation of precursor cells to adipocytes as well as cells for other purposes. These adipocyte precursor cells are, however, not irreversibly determined to adipocytes until available adipocytes are "full", having reached a critical size. The new adipocytes are then filled with fat quickly and disappear among the other fat cells. Presumably this chain of events is occurring through two signal systems, one triggering the formation of new cells in adipose tissue. This signal is presumably generally active and present in the circulation. The finding of variations in stimulation to adipocyte formation of different sera, as tested in vitro, might be caused by such a signal factor. Another signal is presumably triggering the determination of new adipocytes from the pool of precursor cells. It is likely that this signal is active locally within adipose tissue and associated with cellular enlargement. This system is now possible to use in studies of obesity in man, where both possible cellular defects or errors in regulation of adipose tissue cellularity can be studied. This is the plan for next years's work. As biproducts to this project endothelial cells from capillaries and, probably, brown adipose tissue cells have been isolated and cultured. These cells are of potential importance for reconstruction of the physological compartmentalization of lipoprotein lipase activity in adipose tissue, respectively might facilitable studies on the function of brown adipose tissue.